Retroreflective materials are characterized by the ability to redirect light incident on the material back toward the originating light source. This property has led to the widespread use of retroreflective sheeting for a variety of traffic and personal safety uses. Retroreflective sheeting is commonly employed in a variety of articles, for example, road signs, barricades, license plates, pavement markers and marking tape, as well as retroreflective tapes for vehicles and clothing.
Two known types of retroreflective sheeting are microsphere-based sheeting and cube corner sheeting. Microsphere-based sheeting, sometimes referred to as “beaded” sheeting, employs a multitude of microspheres typically at least partially embedded in a binder layer and having associated specular or diffuse reflecting materials (e.g., pigment particles, metal flakes or vapor coats, etc.) to retroreflect incident light. Cube corner retroreflective sheeting typically comprises a thin transparent layer having a substantially planar front surface and a rear structured surface comprising a plurality of geometric structures, some or all of which include three reflective faces configured as a cube corner element.
Cube corner retroreflective sheeting is commonly produced by first manufacturing a master mold that has a structured surface, such structured surface corresponding either to the desired cube corner element geometry in the finished sheeting or to a negative (inverted) copy thereof, depending upon whether the finished sheeting is to have cube corner pyramids or cube corner cavities (or both). The mold is then replicated using any suitable technique such as conventional nickel electroforming to produce tooling for forming cube corner retroreflective sheeting by processes such as embossing, extruding, or cast-and-curing. U.S. Pat. No. 5,156,863 (Pricone et al.) provides an illustrative overview of a process for forming tooling used in the manufacture of cube corner retroreflective sheeting. Known methods for manufacturing the master mold include pin-bundling techniques, direct machining techniques, and techniques that employ laminae.
For example, U.S. Pat. Nos. 3,684,348 and 3,811,983 describe retroreflective material and a method of making a composite material wherein a fluid molding material is deposited on a molding surface having cube corner recesses and a preformed body member applied thereto. The molding material is then hardened and bonded to the body member. The molding material may be a molten resin and the solidification thereof accomplished at least in part by cooling, the inherent nature of the molten resin producing bonding to the body member thereof. Alternatively, the molding material may be fluid resin having cross-linkable groups and the solidification thereof may be accomplished at least in part by cross-linking of the resin. The molding material may also be a partially polymerized resin formulation and wherein the solidification thereof is accomplished at least in part by polymerization of the resin formulation.
A slot die having a certain design has been used to cast molten polycarbonate onto a microprismatic tool surface for the production of retroreflective sheeting including retroreflective sheeting commercially available from 3M Company (“3M”), St. Paul, Minn. under the trade designation “3M Scotchlite Brand Reflective Sheeting 3990 VIP”. The tool surface employed for making retroreflective sheeting comprises a plurality of recesses coming to a sharp point. As the line speed of the advancing tool under the die increases, incomplete filling of the recesses becomes evident as visible microscopic defects. Frequent and/or substantial incomplete filling of the recesses can result in diminished retroreflective performance.
A solution to this problem has been to increase the pressure at the coating die to force the fluid polymer into the tool surface. An extruder has been used to provide the molten polymer at the slot orifice at high pressures ranging up to 5000 psi (pounds per square inch). However, there are limits to this approach. The weight and bulk of the equipment needed to contain such high pressures becomes unwieldy. Further, the increased polymer pressure often causes leakage between the mating halves of the die resulting in production problems such as charred and degraded polymer dripping onto the tool, which damages the tool.
Accordingly, industry would find advantage in improved methods of making retroreflective sheeting and other microstructured articles that employ casting fluid synthetic resin onto a tool as well as a novel slot-die apparatus suitable for such methods of manufacture.